Testing systems used in the prior art tend to test the sealing performance of the entire connection system and do not characterize the sealing performance of the metal-to-metal seal alone. It will be understood that the term metal-to-metal seal is known in the art to include annular seals formed in a region where an interval of continuous contact is provided between mating solid components of similar stiffness wherein a thin film of material may be disposed in the interfacial region of the contacting interval, the thin film of material typically comprised of lubricant film, compressed thread compound paste, and/or surface coating carried by one or both of the mating components.
By way of example, with U.S. Pat. No. 3,653,254 (Simon) entitled “Leak-testing internal seals in pipe joints” the test fluid media is applied externally with a driving differential test pressure that must force sufficient test fluid to pass through the connection thread to first reach the internal metal-to-metal seal and subsequently continue to flow so as to increase the differential pressure across the seal to substantially that of the driving pressure, so as to meet the condition for detection of leakage under a known pressure differential. The connection threads are coated with a thread compound paste to provide lubrication and prevent galling. This thread compound paste typically contains solids and has plastic and thixotropic fluid phase properties. When trapped between thread surfaces it can thus restrict or impede the flow of the externally applied fluid when the clearance between thread surfaces is small. Thus, this method may not reliably evaluate the sealing performance of the metal-to-metal seal separate from the sealing performance of thread compound trapped within the thread, i.e., the driving pressure for leakage past the metal-to-metal seal may not be reliably known as it can depend on the pressure drop of fluid across the thread due to both time dependent viscous characteristics or blockage.
This and other similar test methodologies characteristically provide pass/fail characterization of seal integrity and do not lend themselves to the quantitative definition of seal capacity relative to applied loads.